Evaluation of an Ensemble Dispersion Calculation


Roland R. Draxler


Journal of Applied Meteorology,


Abstract - A Lagrangian transport and dispersion model was modified to generate multiple simulations from a single meteorological data set.  Each member of the simulation was computed assuming a ± one grid point shift in the horizontal and a  ±250 m shift in the vertical of the particle position with respect to the meteorological data.  The configuration resulted in 27 ensemble members.  Each member was assumed to have an equal probability.  The model was tested by creating an ensemble of daily average air concentrations for three months at 75 measurement locations over the eastern half of the United States during the Across North America Tracer Experiment (ANATEX).  Two generic graphical displays were developed to summarize the ensemble prediction and the resulting concentration probabilities for a specific event: a "probability exceed" (PE) plot and a  "concentration probability" (CP) plot.  Although a cumulative distribution of the ensemble probabilities compared with the measurement data was favorable, the resulting distribution was not uniform. This was attributed to release height sensitivity.  The trajectory ensemble approach accounts for about 41% to 47% of the variance in the measurement data. This residual uncertainty is caused by other model and data errors not included in the ensemble design.


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